I'm going to rent a server with 12 SATA disks hardware raid(planned to be configured as raid5), and I'm wondering what's the best practice where to put system partition in that case and the most efficient configuration. I'm going to use the server to serve static files, and the main reason for getting such a server is that I need lot's of space.
Totally impossible to answer. If that is going to be ONE raid 5 (which borders stupid if you ask me - I would at least make it raid 6, 12 discs it too many for a raid 5 in my eyes) then the only logical sense is "on all discs", as a separate first raid group. Otherwise you loose space (as the discs then have different free space). A Raid 10 or Raid 6 for the system is the only logical way to go.
Unless that is a pure file store, then the layout is disputable - if that is a database, I would split out some discs so I don't loose log and data at the same time.
I would say that is the nice time to put in a small SSD for the operating system. Many servers support that. But if not - well ;) A system partition, Raid 6 on all discs.